1. Field of the Invention
The present invention relates generally to the field of optical systems and more particularly to a multiple field of view reflective telescope.
2. Description of Related Art
A reflective (catoptric) telescope with a wide field of view is commonly used today in space-based surveillance sensors. One of the first types of these reflective telescopes that were developed to address the multi-spectral large aperture requirements of space-based surveillance sensors is known as the three mirror anastigmatic optical system. FIG. 1 (PRIOR ART) illustrates one embodiment of a three mirror anastigmatic optical system 100 that is disclosed in U.S. Pat. No. 3,674,334. The three mirror anastigmatic optical system 100 includes an entrance pupil 102 where incident beams 104 from an object (not shown) pass through and are reflected by a primary mirror 106 which causes the beams 104 to converge to beams 108 and form an intermediate image 110. The beams 108 are incident on a secondary mirror 112 which reflects divergent beams 114. The beams 114 are incident on a tertiary mirror 116 which reflects and substantially collimates beams 118 to an exit pupil 120. A more detailed discussion about this embodiment and other embodiments of the three mirror anastigmatic optical system 100 is provided in U.S. Pat. No. 3,674,334 the contents of which are incorporated by reference herein.
The main problem with the three mirror anastigmatic optical system 100 is that it is often too large to be effectively used in a space-based surveillance sensor. To address this problem, John M. Rodgers added a fourth mirror to the three mirror anastigmatic optical system 100 which enabled one to fold the optical design into a more compact arrangement. FIG. 2 (PRIOR ART) illustrates the four mirror anastigmatic optical system 200 invented by John M. Rodgers and patented in U.S. Pat. No. 5,309,276 the contents of which are incorporated herein. The four mirror anastigmatic optical system 200 includes an entrance pupil 202 where incident beams 204 from an object (not shown) pass through and are reflected by a primary mirror 206 which causes the beams 204 to converge to beams 208. The beams 208 are incident on a secondary mirror 210 which reflects convergent beams 212. The beams 212 are incident on a tertiary mirror 214. Beams 216 are reflected from the tertiary mirror 214 to a fourth mirror 218 which substantially collimates beams 220 to an exit pupil 222. It should be noted that beams 216 form an intermediate image 215 just after the tertiary mirror 214. As can be seen, the tertiary mirror 214 is disposed at a large tilt angle (for example 43°) with the incident beam 204, thereby providing a beam folding configuration wherein reflected beams 216 and 220 pass in front of the primary mirror 206 twice. This provides a beam folding function between the entering and exiting beams that allows an extremely compact system configuration. Unfortunately, the four mirror anastigmatic optical system 200 does not support multiple field of views. This need and other needs are satisfied by the multiple field of view reflective telescope of the present invention.